Three-axis seismometer

ABSTRACT

A three-axis seismometer in which three masses are suspended for essentially frictionless movement within certain directions in magnetic fields. Detecting means sense movement of the masses and generate signals used to actuate feedback means. The amount of feedback required is used as a measure of seismic motion. Since motion sensing and the measurement of the magnitude of the motion are separated, output of the instrument may be linear. Responses in unwanted frequency ranges may be filtered out.

United States Patent [72] inventors Ivan Simon FOREIGN PATENTS g i a simm L l Mam L 1,173,266 7/1964 Germany 340/17 ar m an, ex ngton; n Cohen,Belmont; Richard S. Stone, QTHER REFERENCES Lexington a" of Mass IBMTechnical Disclosure Bulletin Vol. No. 4, No. 5, 10/61 [211 App! No.84!,876 by N. D. Anderson Recording Vibrations and Shocks." Filed July1969 Primary Examiner-Rodney D. Bennett, Jr. Patented 1971 AssistantExaminer-N. Moskowitz [73] Assignee Arthur D. Little, lnc.Anorney-Bessie A. Lepper Cambridge, Mass.

ABSTRACT: A three-axis seismometer in which three masses 1 THREE-AXISSEISMOMETER are suspended for essentially frictionless movement within19 Claims, 15 Drawing g certain directions in magnetic fields. Detectingmeans sense I 340 17, movement of the masses generate Signals usedactuate [52] U s C 73/0 feedback means. The amount offeedback requiredis used as a [51] Int. Cl G0lv l/l6 measme of seismic mofion' Sincemotion Sensing and the 501 Field of Search 340/17 11- magniwde 244/321.73/70 put of the instrument may be linear. Responses in unwantedfrequency ranges may be filtered out. [56] References Cited UNITEDSTATES PATENTS 3,480,908 1 H1969 Codina 340/17 4| 2 I22 I23 45 47 8| 5|re2 62 ea 99 1 o 67 b 42 92 63 94 98 95 65 64 II? 96 97 24 127 g 3 I2586 44 1|2| I22 1| I0 I23 SHEET 1 OF 9 Ivan Simon Carl R. SmaIlrnanMartin L. Cohen Richard S. Srone INVENTORS fl' I Attorney FATENTEUUEI:'IISTI Flg 1 Fig. 3

PATENTED use nan 3 62Q354 Martin L. Coh Richard S. S e

INVENTORS Attorney PATENIEUnEc Hem 3.626364 SHEET 0F 9 TO RECORDER lvunSimon Carl R. Smollmon Martin L. Cohen Richard 8. Stone INVENTORSAttbrney PATENTEI] 0E0 7 |97l SHEET 6 OF 9 Ivon Simon Carl R. SmollmonMartin L. Cohen Richard 8. Stone lNVENTORS Attorney SEGMENT 25 SEGMENT26 SEGMENT 24 SHEET 9 UF 9 PATENTED on: 71974 INVENTORS lvon Simon, CarlR. Smollmon,

Attorney Martin L. Cohen, Richard 8. Stone Fig. 15

THREE-AXIS SEISMOMET'ER This invention relates to a seismometer and moreparticularly to a small, three-axis, long-period seismometer which maybe installed and left unattended for long periods of time.

With few exceptions all seismometers consist of a mass suspended byspring or hinges. The mass is arranged to stay essentially stationary inspace as the suspending frame moves with the motion of the earth.Long-period instruments of this type are, of necessity, relatively largeand require a considerable amount of auxiliary equipment. Suchinstruments, although very accurate, must be installed in shallow earthvaults because of their size and shape. This in turn means that wheninstruments of this nature are operated near the surface, as they mustbe, they are limited in their performance by having to cope withrelatively high-level ground noises at low frequency. To avoid thisdifficulty it is desirable to place longperiod seismometers in deepboreholes, e.g., those which are at least 500 feet deep. However, thisis impossible in the case of conventional long-period seismometers, afact which in turn has led to recent developments in the development ofspecific borehole seismometers. However, the present seismometers ofthis type require boreholes 1 l to 12 inches in diameter. Since the costof a borehole is directly related to its diameter, it would be desirableto have a long-period, continuously reliable seismometer which could beinstalled in boreholes of considerably smaller diameter, e.g., about to6 inches.

In addition to their use in small diameter boreholes, there are manyother demands for small seismometers which are extremely sensitive,which can be relied upon to operate continuously over a long period oftime with little or no attention, and which require a minimum amount ofauxiliary equipment. The three-axis, long-period seismometer of thisinvention is capable of filling such demands and is particularlysuitable for a small diameter seismometer which can be placed in deepboreholes.

It is therefore a primary object of this invention to provide a noveland improved small diameter seismometer capable of detecting seismicmotion in three axes. It is another object of this invention to providea seismometer of the character described which is suitable for insertionin deep boreholes for connection through a minimum number of leads withauxiliary electronic equipment which may be placed at any suitableremote place. It is another object of this invention to provide athree-axis, long-period seismometer which is extremely sensitive andwhich at the same time is capable of providing a linear readout. It isyet another primary object of this invention to provide a seismometer ofthe character described which is adapted for a wide range of uses in awide range of environments. It is another object of this invention toprovide an instrument of the character described which is insensitive topressure, gravity, and many other physical influences which affect theperformance of the prior art seismometers. Other objects of theinvention will in part be obvious and will in part be apparenthereinafter.

The invention accordingly comprises the features of construction,combinations of elements and arrangements of parts which will beexemplified in the constructions hereinafter set forth and the scope ofthe invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the inventionreference should be had to the following detailed description taken inconnection with the accompanying drawings in which FIG. 1 is asimplified diagrammatic representation of the three levitated masses,the movements of which along their axes are used to measure seismicmotions;

FIG. 2 is a perspective side view, partly cut away, of a threeaxisseismometer constructed in accordance with this invention having thethree segments in axial alignment;

FIG. 3 illustrates a modification in the positioning arrangement of thethree segments making up the seismometer of this invention;

FIG. 4 is a perspective view, with some details omitted, of one of thesegments having a horizontally oriented mass;

FIG. 5 is an enlarged cross section of a portion of the magnetic fieldgenerating means employed in the segment of FIG.

FIG. 6 is an enlarged somewhat diagrammatic view showing a portion ofthe detecting means used;

FIG. 7 is a detailed circuit diagram of the embodiment of a portion ofthe circuitry associated with the seismometer segment of FIG. 4;

FIGS. 8 through 11 are diagrams of additional feedback systemembodiments, those of FIGS. 8-10 using magnetic forces and that of FIG.11 using electrostatic forces;

FIG. 12 is a perspective view, with some details omitted, of theseismometer segment having a vertically oriented mass;

FIG. 13 is a longitudinal cross section of the seismometer segment ofFIG. 12 showing its detailed construction;

FIG. 14 is a detailed circuit diagram of one embodiment of a portion ofthe circuitry associated with the seismometer segment ofFIG. 11; and

FIG. 15 is a diagrammatic representation of the entire apparatusincluding recorder and readout means.

The three-axis, long-period seismometer of this invention is formed inthree segments, two of which have horizontally oriented elongateddiamagnetic masses supported in magnetic fields designed to permitessentially only axial movement of the diamagnetic mass, and the thirdsegment of which comprises a magnetically supported mass afiixed to avertically oriented elongated member. The horizontally oriented massesare used to detect and measure seismic motions in what for conveniencemay be designated the north-south and east-west directions; while thevertically oriented mass measures seismic motion in the verticaldirection. A feedback system is associated with each of these masses andis used to constrain the relative motion of each mass and its associatedpart of the apparatus in contact with the earth, the seismic movement ofwhich is to be determined. The amount of feedback required for suchconstraint is a measure of the amount of motion which could have beenexperienced by each mass and hence is a measure of seismic motion ineach of the three axes. Since the amount of feedback is measured interms of current or voltage required to effect the necessary constraint,and since it can, if desired, be made linear, the measurements ofseismic motion may be linear without in any way lessening thesensitivity of the detection of the motion of the masses. Hence, thethree-axis seismometer of this invention may, if desired, attain whathas heretofore not been considered possible-it can couple extremesensitivity with linear output.

FIG. 1 shows in diagrammatic fashion the three masses, 20, 21, and 22,used in the three seismometer segments 24, 25, and 26. Masses 20 and 21sense and measure seismic motion in the two directions in the horizontalplane, e.g., the east-west and north-south directions; while mass 22senses and measures seismic motion in the vertical direction. The orderof segment arrangement of FIG. 1 may be changed, e.g., the verticallyoriented mass segment 26 may be on the bottom or between the twosegments 24 and 25.

The embodiment of the apparatus of this invention shown in FIG. 2 isparticularly suitable for insertion in boreholes. The three segments,24, 25, and 26, containing the suspended masses, are positioned within amain cylindrical housing 30, the outside diameter of which is slightlysmaller than the diameter of the borehole to be used. The segments 24,25, and 26 are held firmly together in spaced relationship by acombination of spacers, such as the threaded turnbuckles 31 and twocollars 32 and 33 fitted around the ends of the adjoining segments.Another threaded turnbuckle (not shown) joins segments 24 and 25. Thesecollars are relatively thick annular rings having slots, such as slot 34shown for collar 32, running the entire length of the collar. Thesecollars 32 and 33 position and align the segments 24, 25, and 26 withinthe housing 30, and define between the segment housings and the innerwall of the housing 30 a series of annular passageways 36 which providesuitable spacings for cables 37, 38, and 39 (carrying the lead wiresconnecting with remote equipment) to be positioned between the innerwall of the housing 30 and the outer surface of the segment housings.The slots 34 and annular passageways 36 permit the cables to passthrough an opening 40 in the top of the main housing 30. Cable 37associated with the lowest segment 24 would normally lie within theconfines of the slot in collar 33. However, in the drawing in FIG. 2 ithas been drawn to be separated so that the threaded turnbuckle 31 may beshown.

FIG. 3, in which like numbers refer to like components in FIG. 2, showsanother arrangement of the three segments in which they may bepositioned side by side, either directly on the ground or in a housing27. This, of course, is satisfactory if they are to be placed in a vaultor in some similar location, but this arrangement cannot be used inboreholes.

The seismic motion in the east-west or north-south direction is sensedby using a diamagnetic mass suspended in a magnetic field of aparticular flux configuration which is' such as to constrain the bodyradially but permits it to move axially within certain predeterminedlimits. The suspension of the diamagnetic mass in this manner isessentially frictionless, and the apparatus may be constructed to berelatively sturdy and stable over an extended period of time. Inasmuchas the physical phenomena of diamagnetism and magnetism do not dependupon temperature (below the Curie point of the materials), gravity orother physical influences, the horizontal seismometers may be operatedover a very wide range of physical conditions. Any axial shift in thediamagnetic mass brought about through the effective displacement of theinstrument is measured in the form of the amount of feedback required tomaintain the mass within predetermined limits. This requires some meansfor restoring the mass to its pull position. As will be shown in thedetailed description to follow, the horizontal seismometers of thisinvention also incorporate installation leveling means which may beactuated either manually or mechanically.

FIGS. 4-1 1 are directed to illustrating various embodiments of theconstruction and to describing the operation of segments 24 and 25 whichcontain horizontally oriented diamagnetic masses. These segments 24 and25 are identical in construction. FIG. 4 is a perspective view of one ofthese segments with a portion of its housing cut away; FIG. is anenlarged detailed cross-sectional view showing the manner in which thediagmagnetic mass is suspended in a magnetic field; and FIG. 6illustrates the functioning of a portion of the detecting means.Reference should be had to FIGS. 4-6 in the following description inwhich seismometer segment 24 is used to illustrate the measurement ofseismic motion in the east-west or north-south direction.

The segment 24 is enclosed within a housing 41 which serves as amagnetic shield. This segment may be considered to be made up of threeintegrated components, namely the main horizontal seismometer 42, theleveling mechanism 43 and the feedback tilter 44.

The manner in which a diamagnetic mass is suspended in a magnetic fieldmay be explained with reference to FIG. 5 which illustrates how a massof a diamagnetic material, such as an elongated cylinder 45, can besuspended in a properly designed magnetic field 46. In order to achievethe desired levitation of the mass 45, it is necessary to provide amagnetic field which has a vertical gradient decreasing upwardly andwhich exhibits symmetrical transverse gradients which are substantiallyuniform. In FIG. 5 the lines of flux have been drawn in to illustratethe flux gradient and from these lines it will become apparent that themagnet is so arranged as to strongly constrain the diamagnetic mass in atransverse direction while leaving it free to move with essentially nofriction in the axial direction. As will become apparent in thefollowing detailed description, the axial movement is constrained andthe magnitude of the signal (e.g., the amount of voltage or current)required to effect such constraint becomes a measure of the deviationthat the suspended mass would normally undergo from a true horizontalplane.

The desired magnetic field is achieved by use of an upper pole piece 47in which there is a groove 48 having edges 49 and 50 which, according towell-known physical principles, effect a concentration of the magneticflux. A lower pole piece 51 is provided and in the modification of FIG.5 is seen to terminate in a narrow, flat surface 52 which provides edges53 and 54 so aligned with edges 49 and 50 as to achieve the desiredlateral flux gradient as well as a vertical gradient. It will beappreciated that these edges need not be sharp but may be rounded tooptimize magnetic saturation in the pole piece material.

As seen in FIG. 4, the pole pieces 47 and 51 are affixed to twointegrated permanent magnets 60 and 61. These in turn are mounted in ahorizontally positioned seismometer frame 62 which on one end has arelatively thick vertical piece 63 serving as a means to position twovertical support members 64 and 65. The diagmagnctic mass 45, typicallya rod of graphite, has afiixed to one end a thin lightweight rod 66which serves as a support for lightweight, light-obstructing vane 67,which is part of the detecting system shown in somewhat enlargeddiagrammatic detail in FIG. 6. An end stop 68 is provided to engage theend of rod 66 to limit the travel of the diamagnetic mass (FIG. 6). Asimilar end stop (not shown) is associated with the other end of themass.

Affixed to vertical support 64 is a miniature light bulb 70 providing asource of radiation. Affixed to vertical support 65 is a dualphotoresistive photocell 71. The radiation source 70, thelight-obstructing vane 67 and the photocell 71 are so arranged that whenthe apparatus is positioned to be truly horizontal the vane 67 dividesthe radiant energy striking the photocell equally between the two sidesof the photocell. As will be seen from FIG. 6, if the position of thevane shifts relative to the position of the photodetector by virtue ofthe fact that the frame 62 has moved with the earth, the amount ofradiation falling on the light-sensitive elements 72 and 73 will bedifferent. This difi'erence is then reflected in the amount of currentdelivered to an amplifier 75 through the circuit illustrated. The use ofthe signal derived from amplifier 75 will be explained in detail inconnection with the description of the feedback mechanism for thesegment. Lead wires (shown as a single wire) connect the lamp to asuitable power source; while lead wires 81, 82, and 83 connect cell 71with its as sociated circuit. These lead wires are taken through cable37 for connection to equipment as described in connection with FIG. 7.

Inasmuch as the apparatus of FIG. 4 is particularly suitable forincorporation in a three-axis seismometer to be inserted into a deepborehole, it is advantageous to have means for leveling the seismometerafter installation, the actual leveling being controlled at a locationquite remote from the location of the seismometer itself. The horizontalseismometer of FIG. 4 is equipped with leveling means 43 which may bemanually or automatically operated. The seismometer frame 62 issupported on a tiltable platform having a groove in its bottom surfaceadapted to engage a pivot pin 91 for pivotal motion along an edge topermit alignment of the seismometer frame 62. The pivot pin 91 in turnrests in another corresponding groove in the vertical component 92 of anintermediate support system 86 illustrated in FIG. 4 to be formed ofplates 87 and 88. The actual pivoting of the platform 90 is achievedthrough a nut 94, and a fine-pitch screw 95 which in turn is driven by agear 96 and worm 97, the latter being turned by a leveling motor 98which is connected through lead wires 99 to signal-control means shownin FIG. 7 as motor control circuit 101. The motor 98 is convenientlysupported in an auxiliary vertical support 100. By manually orautomatically operating the motor 98, it is possible to adjust theplatform 90 by pivoting it on pivot pin 91 and hence to level thediamagnetic mass 45. Through the use of the motor control circuit 101 inFIG. 7, automatic operation (leveling) is accomplished by operation ofpolar relay 102 through threshold device 103 when the output signalapproaches the maximum value that the circuits can provide. The contacts104 and 105 of this polar relay 102 are connected in parallel with thoseof the manual leveling switch 106 and thus automatic leveling isprovided whenever a large deviation from the null position exists.

The feedback tilter system 44 illustrated in FIG. 4 is one whichincorporates piezoelectric crystals to automatically adjust platform 90with respect to the position of diamagnetic mass 45. This isaccomplished by interposing a piezoelectric system between theintermediate support system 86 and the base plate 110 through brassrings 11 l and 112. The baseplate 110 is, in effect, connected to theground, the seismic motion of which is to be measured. This connectionis normally through the structure illustrated in FIG. 2.

In the embodiment of FIG. 4 the feedback tilter is a piezoelectricthree-point suspension system comprising three piezoelectric assemblies,namely the two front assemblies 115 and 116 and the single back assembly117. Each of the two front assemblies, 115 and 116, are, for simplicityof illustration, shown to be fonned of two piezoelectric crystals, 118and 119, having an electrode 120 located between them. These twopiezoelectric crystals are connected for grounding by wires 121. and arepoled so that they contract when a voltage is applied to the electrode120 connected to appropriate circuitry (described below) through leadwires 122 and 123. The single-back piezoelectric assembly 117 islikewise shown in its simplest form to comprise two piezoelectriccrystals 124 and 125 with an electrode 126 and grounding wire 127. Thetwo front electrodes I20 and back electrode 126 are connected throughlead 128. The piezoelectric assemblies 115, 116 and 117 may be formed ofany even number of crystals in accordance with well-known techniques.Thus if the front assemblies 115 and 116 are poled to contract whenvoltage is applied to electrode 120, the back assembly crystals will bepoled to expand when voltage is applied to electrode 126.

In a similar manner the piezoelectric crystal can be arranged to impartlateral, rather than angular, motion to frame 62. In such amodification, the relative motion of the seismic mass with respect tothe field generating means is restored by inertial forces rather than bygravitational forces as in the arrangement of FIG. 4.

Signals received from the amplifier 75 control, through a feedbackcircuitry to be described, the amount of voltage which is delivered tothe electrodes 120 and 126, and hence control the orientation of theplatform mechanism on which the seismometer rests. Lead wires 122 and123, along with lead wires 80, 81, 82, 83, and 99, are combined in asingle cable 37 for connection with the electronic components to bedescribed below.

FIG. 7 is a diagram for the feedback circuit of the apparatus of FIG. 4.Like numbers refer to like components in FIGS. 4 and 6. As explained,the photodetecting system, such as shown in FIG. 6, measures the amountof seismically induced motion in the frame and its supporting systems.The purpose of the feedback mechanism is to constrain the motion of thediamagnetic mass and this is done by shifting the position of the frameand hence the position of the magnetic field generating means, e.g., themagnets and pole pieces, relative to the freely suspended diamagneticmass to that it will remain in its null position. The voltage requiredto efiect this constraint is a measure of the seismic motion. Therefore,in the case of the apparatus of FIG. 4, the voltage input to itsassociated amplifier 141 (described below) is the measure of the seismicmotion.

One exemplary circuit capable of automatically effecting the requiredconstraint is shown in the embodiment of FIG. 7. The signal transmittedvia lead wires 81, 82, and 83 from the photodetector system (FIG. 6) isfed to a preamplifying circuit 130 which includes amplifier 75. Thissignal may be visually monitored, if desired, through a monitoringcircuit 131 including a switch 132 and microammeter 133. It is desirableto incorporate a response-adjusting circuit 135 (which includes anamplifier 136) for error-rate damping, maintaining stability or forotherwise modifying the overall response prior to its transmittal to thevoltage amplifier circuit 140 which in turn ineludes an amplifier 141and switch 142. Inasmuch as the voltage transmitted to the voltageamplifier circuit is a measure of the seismic motion, this voltage alsobecomes the signal delivered to a recorder and any other signal-actuatedmeans as described in conjunction with FIG. 15. Such signal-actuatingmeans are attached through switch 145 and line 146. The voltageamplifier circuit provides a voltage which is proportional to the signalreceived from the photodetector system and this signal is used tocontrol the voltage input to electrodes 120 and 126 of the piezoelectricsystem. Since the crystals of piezoelectric assemblies 115 and 116 arepoled to contract when voltage is applied and piezoelectric assembly 117is poled to expand under the same condition, the signal from the voltageamplifier circuit controls the degree to which the intermediate supportsystem 86 is adjusted to maintain seismometer frame 62 in the desiredposition relative to diamagnetic mass 45. Thus, for example, if theseismic motion is such as to displace frame 62 with respect to the mass45 in a given direction, then the feedback circuit provides a voltage tothe electrodes of the piezoelectric assemblies which tilts the frame byan amount necessary to allow gravity to restore the relative position ofthe mass and frame.

While the voltage amplifier and piezoelectric assembly system of theapparatus of FIG. 4 are designed to restore the relative position of thediamagnetic mass with respect to the frame by tilting the entireassembly, the embodiments illustrated in FIGS. 8-11 are designed toapply magnetic or electrostatic forces to the diamagnetic mass itself torestore it to a null position and to use the amount of this forcerequired as a measurement of the seismic motion experienced. Theembodiments of FIGS. 8 and 10 use magnetic force while that of FIG. 11uses electrostatic force.

In the embodiment of FIG. 8 the pole pieces and photodetecting means arethe same as in FIG. 4. Affixed across each end of the pole pieces 47 and51 is an iron plate 151 held by upper nonmagnetic spools 152 and lowernonmagnetic spools 153. Coils 154 and 155 are wound around the upperspools 152 and coils 156 and 157 are wound around spools 153. Inoperation, a signal from the photodetector system is delivered toamplifier 75 (FIG. 7) and returned to the seismometer. However, in theembodiment of FIGS. 8 and 9, the returning signal takes the form ofcurrent to the coils. If for example the diamagnetic mass 45 has movedaxially to the right, the feedback signal is in the form of additionalcurrent to coils 154 and 156. The magnetic field in the area designated158 is strengthened, the diamagnetic mass is repelled and caused toshift toward the left to its null position. At the same time current isapplied to coils 155 and 157 to weaken the magnetic field in the areadesignated 159, thereby enhancing the effect. In like manner the currentsignal to coils 154, 155, I56, and 157 can be reversed to effect aconstraining force when the mass attempts to shift to the left. Theseismometer of FIGS. 8

' and 9 may be positioned on a level-adjustable platform and used as inFIG. 4. However, the intermediate platform system 86 and piezoelectricsystem are eliminated.

In the embodiment shown diagrammatically in FIG. 10 the restoring forceis magnetic and the detecting system is modified to include two separatephotocells and to eliminate the light-obstructing vane.

In the device of FIG. 10, the optical portion of the detecting meansincludes a radiant energy source and two mirrors 176 and 177 alignedwith photocells 178 and 179. The lower pole piece 51 is modified ateither end to include electromagnets 182 and 183 having coils 184 and185, respectively. In like manner, the upper pole piece 47 may be somodified to include electromagnets in addition to or in place of theelectromagnets 182 and 183 associated with the lower pole piece.

Coil 184 is connected to a circuit which includes a current amplifierand resistor 191; and in like manner, coil 185 is part of a circuitincluding current amplifier 192 and resistor 193. Photocell 178 isconnected to a differential amplifier 194 which has a feedback loopcomprising capacitor 195 and resistor 196 in parallel; and photocell 179is connected to a differential amplifier 197 which has a feedback loopcomprising capacitor 198 and resistor 199 in parallel. It will be seenthat differential amplifier 194-is also connected to current amplifier192 and coil 185; while differential amplifier 197 is connected tocurrent amplifier 190 and coil 184, the connections between thedifferential amplifiers and the coils providing current feedback loops.A DC current is provided to the amplifiers in the usual manner from asource not shown.

In the operation of the seismometer of FIG. 10, currents are passedthrough both coils 184 and 185 in a direction such as to reinforce thefields generated by the permanent magnets associated with the polepieces. The fields at the ends of the pole pieces are thus made strongersimilar to the situation shown in FIGS. 8 and 9. Now, if one of thecurrents in either coil 184 or 185 is made stronger, the stronger fieldgenerated by the corresponding electromagnet will push the diamagneticmass toward the center of the suspension; similarly, making the currentweaker will pennit the diamagnetic mass to move outwards, away from thecenter.

If the seismometer is displaced so that the mass 45 shifts, say, to theright, the left-hand photocell 179 receives more light, and theleft-hand differential amplifier 197 generates larger output voltage.This voltage is fed into the right-hand current amplifier 190 causing itto pass stronger current through the coils 184 on the right and the massis constrained. In a manner similar to that described for the operationof the system of FIG. 8, this effect is enhanced by weaking the field onthe other end.

The current feedback loop delivers a voltage proportional to theconstraining current to the other input of left-hand differentialamplifier 197 and causes the mass promptly to reach the initial zeroposition to be held there as long as there is any displacement of theseismometer. As in the other embodiments illustrated, the amount ofconstraining current is a function of the seismic motion and it may beread from milliammeter 200 and fed to a chart or computer by line 146 asdescribed below in conjunction with the discussion of FIG. 15.

The embodiment FIG. 11 employs a capacitance displacement detectionsystem and an electrostatic constraining force resulting from applying avoltage to electrodes located near the ends of the diamagnetic mass. Inthe arrangement of FIG. 11 two modified pole pieces 210 and 211,designed to bring about a more gradual change of the flux gradient byreason of their concave configuration, are arranged to cause therequired constraining force to increase proportionally with thedisplacement of the diamagnetic mass from null position. The diamagneticmass 45 is a graphite rod 212 with a copper sheath 213. Interposedbetween the mass 45 and the pole pieces are two end cylindricalelectrodes 215 and 216 and a central electrode 217, this last electrodebeing connected to ground 218. Electrostatic forcer electrodes 219 and220 are positioned within the terminal electrodes 215 and 216, and theyare made part of a feedback system which functions similarly to that ofFIG. 10. This feedback system is comprised of an AC source 221 and acapacitor 222 and differential amplifier 223 associated with terminalelectrode 215; and of a capacitor 224 and differential amplifier 225associated with terminal electrode 216. The differential amplifiers 223and 225 are provided with feedback loops comprising resistors 226 and227 and capacitors 228 and 229 in parallel which have suitably long-timeconstants.

The charged electrodes 219 and 220, of whatever polarity, will inducecharges of equal size and opposite polarity on the surface adjacent tothe electrodes. Thus, the charging of one of the electrodes will causethe mass 45 to be attracted to that electrode. In this way it ispossible to exert force on the mass in order to counteract its motioncaused by displacement of the seismometer and the amount of the forcerequired is a measure of the seismic motion experienced. Since thedisplacement detection system employs AC current rather than DC, theimbalance in voltage in either side of the system is rectified by diodes230 and 231 before it is applied to the input of the correspondingdifferential amplifier. In the feedback loops of this system, thefeedback voltages are derived from resistive voltage dividers 232 and233 connected to control electrodes 219 and 220; and voltage amplifiers234 and 235 are provided in the electrostatic forcer electrode circuits.A millivoltmeter 236 is illustrated as representative of means forreading out the axial movement of the diamagnetic mass and line 146connects the seismometer to a chart recorder and other signal-actuatedmeans as described in conjunction with FIG. 15.

FIGS. 12-14 illustrate in detail the construction of a preferredembodiment of segment 26, i.e., that seismometer component employed tomeasure seismic motion in the vertical axis. A mass mounted on anelongated rod is suspended in a vertical orientation in a magnetic fieldwhich permits relative motion of the mass and its associated structurealong the axis of the mass. The mass and rod are maintained in alignmentthrough the use of diamagnetic bearings which are essentiallyfrictionless. A photodetector system generates a signal which is used toactuate a feedback system which is magnetic in character.

FIG. 12 is a perspective view of the vertical seismometer segment 26with the housing and fiux tube partly cut away and some componentsremoved; while FIG. 13 is a detailed cross section of the sameapparatus. Reference should be had to both of these drawings in thefollowing description.

The vertical seismometer is positioned within a housing 240 which servesas a magnetic shielding. It may be considered to be divided into fourcomponents, namely the seismic mass and its suspension means 241, anupper diamagnetic bearing 242, a lower diamagnetic bearing 243 and adetector system 244.

In the embodiment of FIGS. 12 and 13 the seismic mass 245 is a permanentmagnet which is encased in a thin-walled stainless steel tubing 246.Suspension of the magnet mass 245 is by means of a magnetic field havinga vertical gradient along the axis of the seismic mass. A reentrantmagnetic (e.g., iron) circuit containing pole pieces shaped in the formof a hyperboloid of revolution creates the required magnetic suspensionfield. The magnetic circuit is made up of an upper pole piece cap 247,an upper permanent ring magnet 248, and upper flux return plate 249,with flux shunt adjustment screws 250, a flux return tube 251, a polepiece ring 252 (centrally positioned within said flux return tube) alower flux return plate 253, a lower permanent ring magnet 254 and alower pole piece cap 255. With the seismic mass and the poles of themagnets oriented in the manner shown, the mass (along with the tubing246 and the components affixed to the ends of the tubing associated withthe diamagnetic bearing) is freely supported by a combination ofattracting and repelling magnetic forces.

Positioned between pole piece caps 247 and 255 is a coilholdingframework piece 256 formed of a suitable nonmagnetic material such as asynthetic resin and shaped in an essentially annular configurationhaving a series of coil-supporting recesses. The first set of suchcoils, i.e., motor coils 257 and 258 are those associated with thefeedback mechanism designed, as in the case of the horizontal seismicmasses, to constrain the relative motion of the mass and the structuresurrounding it. Motor coils 257 and 258 are connected in seriesopposition through lead wires 259 and 260 to the feedback circuitry(FIG. 14) and their purpose is to exert force on the suspending magnetsystem when energized by the feedback mechanism.

The second set of coils, i.e., coils 261 and 262 which are positionedabout the seismic mass may be referred to as period-adjustment coils.They are connected in series opposition by lead wires 263 and 264 to aconstant current source (FIG. 14). The purpose of theseperiod-adjustment coils is to produce a spatially localized gradient inthe field to create a small perturbation which in turn prevents the massfrom being supported in a field of indifierent equilibrium and infiniteperiod. By energizing these coils with a DC current the mass oscillateswith a period which is proportional to the current applied to coils 261and 262.

Associated with each end of the stainless steel tube, which serves as amounting and aligning means for the seismic mass, is a diamagneticbearing assembly. The bearings maintain the tube and hence the mass inprecise vertical alignment while permitting frictionless up and downmovement. Since the upper and lower diamagnetic bearing assembly 242 and243 are identical in construction, the upper diamagnetic bearing 242 maybe described as exemplary.

A diamagnetic mass 270, e.g., a rod of graphite is affixed to the upperend of tube 246. This mass is positioned within an opening 271 definedby a plurality e.g., four) quadrupole pole pieces 272 of quadrupolepermanent magnets 273. These magnets 273 are arranged circumferentiallyto alternate in polarity so as to define within opening 271 a magneticfield, the flux intensity of which decreases rapidly from the surface ofthe magnets to the center of the established field. This then forms adiamagnetic bearing wherein the diamagnetic mass 270, acting as a shaftis maintained in spaced relationship and out of contact with the magnetsurfaces. Since the diamagnetic mass is repelled by the magnetic field,the tube 246 is maintained in axial alignment without any surfacecontact being made between the shaft 270 and the sleeve defined by themagnet surfaces. The quadrupole magnets 273 are held between an upperflange 275 and a lower flange 276 which in turn is supported by acoupling spacer 277 on the flux return plate 249.

In like manner, the lower diamagnetic bearing assembly 243 is formed ofa diamagnetic rod 280 affixed to the lower end of tubing 246, positionedwithin an opening 281 defined by pole pieces 282 affixed to magnets 283.Upper flange 285, lower flange 286 and coupling spacers 287 hold themagnets in position relative to flux return plate 253.

An end stop rod 290 is inserted into the lower diamagnetic rod 280 forengagement with a lower end stop 291 to terminate the downward travel ofthe seismic mass. The end stop is mounted in a spacer plate 292 which isaffixed to a base plate 293, which in turn is sealed to housing 240through an oring and serves as the bottom of housing 240. Baseplate 298in effect contacts the ground, the seismic motion of which is to bemeasured. This contact may be direct; but as in the apparatus of FIG. 2it is usually made through seismometer housing 30.

The seismic motion detector system 244 is located in the upper portionof the apparatus. It is similar in operation and construction to that ofFIG. 6 and is based upon the use of a dual photocell 295, a sourceofradiation such as miniature lamp 296, and a light-obstructing disk 297affixed to an end stop rod 298 which is mounted in diamagnetic rod mass270. Leads 299 (corresponding to leads 82 of FIG. 6) connect thephotocell with a power supply. Leads 300 transmit the detector signal tothe feedback circuit as described below in connection with FIG. 14.Wires 301 connect the lamp 296 to the power source.

The upper end stop rod 298 has an upper end stop 302 mounted in a stopplate 303 which is supported by posts 304 above a terminal board 305from which lead wire 259, 260, 263 and 264 are drawn to be passed withleads 299, 300 and 301 through cable 39.

The feedback circuit associated with the vertical seismometer and shownin FIG. 14 is similar to that associated with the horizontal seismometersegments as illustrated in FIG. 7. Thus in H0. 14 the circuit comprisesa preamplifying circuit 310, with amplifier 311; a response adjustingcircuit 315 with amplifier 316; and a motor coil-driving circuit 320with amplifiers 321 and 322. A switch 324 is provided to connect in arecorder by leads 325, and a switch 326 provides an integrating actionwhich minimizes any initial null position error.

in operation, the seismic mass 245 is positioned in its null positionand then sufficient DC current is put into the periodadjustment coils toestablish a desired period of oscillation to the mass. When the groundwith which baseplate 293 is in contact moves, the apparatus moves whilethe suspended mass tends to remain fixed, thus in effect altering thealignment of light source 296, light-obstructing disk 297 and dualphotocell 295. The signal transmitted by cell 295 is fed back into coils259 and 260 in a manner to constrain mass 245 so as to effectively forceit to remain in its null position, the amount of current required toeffect this mass constraint being proportional to the seismic motionexperienced.

The integration of the circuitry of the three seismometer segments isillustrated diagrammatically in FIG. 15 wherein like components aregiven like reference numbers in FIGS. 2, 7, 12, 13, and 14. in order tosimplify the diagram, only the leads (shown as one line) from thedetecting devices and to the feedback systems are shown along with theleads to the leveling motors for segments 24 and 25.

In addition to the feedback circuitry shown in FIG. 7 the horizontalseismometer segments are shown to have, in series, a first optionaladditional amplifier 330, a filter 331, and a second additionalamplifier 332. The motor control circuit 101 and ammeter 335 are shownto be connected with the leveling motor 98 and it is to be understoodthat either manual or automatic operation of motor 98 is contemplated asdescribed previously.

In a similar manner the circuitry associated with the verticalseismometer segment is shown to include an optional first additionalamplifier 335, a filter 336 and a second additional amplifier 337. Thefilters permit the elimination of responses in undesirable frequencyranges, thus screening the information transmitted to the chart recorder340 as well as to the automatic data processing and storing mechanism344. Seismic motions in all three axes are recorded simultaneously asillustrated by records 341, 342 and 343. At the same time theinformation from each seismometer circuitry is transmitted to amultiplexer 345, then to an analog-to-digital converter 346 and finallyto a suitable digital recorder 347.

The three axis, long-period seismometer of this invention is both sturdyand sensitive. Because it is very compact and requires a minimum numberof connections with any desired recording and readout means, it may belocated in almost any place required. Finally since the seismometer ofthis invention is essentially uneffected by changes in physicalconditions surrounding it, it may be installed and then left unattendedfor extended periods of time.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are efficiently attained and,since certain changes may be made in the above constructions withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

We claim:

11 A three-axis seismometer, comprising in combination a. first andsecond horizontally oriented diamagnetic masses arranged to move indirections perpendicular to each other;

b. first and second magnetic field generating means adapted to supportsaid first and second diamagnetic masses for essentially only axialmovement;

c. a third vertically oriented mass;

. a third magnetic field generating means adapted to support said thirdmass for essentially only axial movement;

e. separate detecting means associated with each of said masses adaptedto sense relative seismic motion of said mass and magnetic fieldgenerating means and to produce a signal; and

f. separate feedback means associated with each of said masses and itsrespective magnetic field-generating means, each of said feedback meansbeing adapted to receive said signal from said detecting means and toconstrain said relative motion between said mass and its magnetic fieldgenerating means, the amount of constraint required being related in apredetermined manner to the magnitude of said seismic motion.

2. A three-axis seismometer in accordance with claim 1 wherein saidconstraint is effected through magnetic, electrollll static, inertial orgravitational forces, or in a combination thereof.

3. A three-axis seismometer in accordance with claim 1 wherein saidfirst and second masses are elongated rod members and each of said firstand second magnetic field-generating means comprises an upper pole piecedefining a horizontal channel and a lower pole piece of a configurationto define with said upper pole piece a magnetic field having a fluxconcentration which is essentially symmetrical along the axis of saidmass and which exhibits vertical and transverse gradients whereby saidmass is strongly constrained transversely but is free to move axially.

4. A three-axis seismometer in accordance with claim 1 wherein saidthird mass is a ferromagnetic body so oriented within a magnetic fieldas to be supported by a combination of magnetic attraction and magneticrepulsion.

5. A three-axis seismometer in accordance with claim 1 wherein saidvertically oriented mass is mounted in a vertically positioned rodmember and diamagnetic bearings are associated with the two ends of saidrod member thereby to maintain it in essentially frictionless alignment.

6. A three-axis seismometer in accordance with claim 1 wherein saidmasses have light-obstructing means associated with one end thereof andeach of said detecting means comprises a source of radiation and a dualphotocell positioned to respond to movement of said light-obstructingmeans.

7. A three-axis seismometer in accordance with claim 1 wherein saidfeedback means associated with said first and second masses comprisemeans to adjust the position of support platform means on which saidfirst and second magnetic field generating means are mounted relative tosaid diamagnetic masses.

8. A three-axis seismometer in accordance with claim 7 wherein saidmeans to adjust the position of said support platform means comprisespiezoelectric crystals interposed between said platform means and basemeans in contact with the ground, the seismic motion of which is to besensed.

9. A three-axis seismometer in accordance with claim I wherein saidfeedback means associated with said masses comprises means to force saidmasses to return to a predetermined position relative to said magneticfield generating means.

10. A three-axis seismometer in accordance with claim 1 wherein saidfeedback means associated with said masses comprises means to change theflux pattern in the magnetic field generating means whereby saidmagnetic masses are returned to a predetermined position relative tosaid magnetic field-generating means.

11. A three-axis seismometer in accordance with claim 1 includingrecording means adapted to record the seismic motion of each of saidmasses as a function of said amount of constraint required to containsaid relative motion between each ofsaid masses and its magnetic fieldgenerating means.

12. A three-axis seismometer, comprising in combination a. first andsecond horizontally oriented diamagnetic masses arranged to move indirections perpendicular to each other;

b. first and second magnetic field-generating means adapted to supportsaid first and second diamagnetic masses for essentially only axialmovement;

0. a third vertically oriented mass;

d. a third magnetic field-generating means adapted to support said thirdmass for essentially only axial movement;

e. separate detecting means associated with each of said masses adaptedto sense relative seismic motion between said mass and said magneticfield generating means and to produce a signal;

separate feedback means associated with each of said masses and itsrespective magnetic field-generating means and adapted to constrain saidrelative motion between said mass and its associated magneticfieldgenerating means; g. separate feedback-actuating means adapted toreceive said signal and to actuate said feedback means whereby saidrelative motion between said mass and its associated magneticfield-generating means is constrained;

h. means to record said signals, which are proportional to said seismicmotions, as a function of time;

i. an outer cylindrical housing; and

j. first, second, and third inner cylindrical housings in axialalignment located in connected spaced relationship within said outercylindrical housing, each of said inner housings containing itsrespective diamagnetic mass, magnetic field-generating means, detectingmeans and feedback means.

13. A three-axis seismometer in accordance with claim 12 wherein saidfeedback means includes means to provide magnetic, electrostatic,inertial or gravitational force to effect the required constraint.

14. A three-axis seismometer in accordance with claim 12 includingremotely activatable levelling means associated with said first andsecond magnetic field-generating means.

15. A three-axis seismometer in accordance with claim 12 wherein saidfeedback means comprise means to force said masses to return to apredetermined position relative to said magnetic field-generating means.

16. A three-axis seismometer in accordance with claim 15 wherein saidforce is magnetic.

17. A three-axis seismometer in accordance with claim 12 includingperiod-adjustment coils associated with said third mass and means tosupply DC current to said coils.

18. A three-axis seismometer in accordance with claim 12 wherein saidfeedback means associated with said third mass comprise motor coilsconnected in series opposition.

19. A three-axis seismometer in accordance with claim 12 wherein saidmeans to record said signals include filter means adapted to filter outunwanted frequencies from said signal.

1. A three-axis seismometer, comprising in combination a. first andsecond horizontally oriented diamagnetic masses arranged to move indirections perpendicular to each other; b. first and second magneticfield generating means adapted to support said first and seconddiamagnetic masses for essentially only axial movement; c. a thirdvertically oriented mass; d. a third magnetic field generating meansadapted to support said third mass for essentially only axial movement;e. separate detecting means associated with each of said masses adaptedto sense relative seismic motion of said mass and magnetic fieldgenerating means and to produce a signal; and f. separate feedback meansassociated with each of said masses and its respective magneticfield-generating means, each of said feedback means being adapted toreceive said signal from said detecting means and to constrain saidrelative motion between said mass and its magnetic field generatingmeans, the amount of constraint required being related in apredetermined manner to the magnitude of said seismic motion.
 2. Athree-axis seismometer in accordance with claim 1 wherein saidconstraint is effected through magnetic, electrostatic, inertial orgravitational forces, or in a combination thereof.
 3. A three-axisseismometer in accordance with claim 1 wherein said first and secondmasses are elongated rod members and each of said first and secondmagnetic field-generating means comprises an upper pole piece defining ahorizontal channel and a lower pole piece of a configuration to definewith said upper pole piece A magnetic field having a flux concentrationwhich is essentially symmetrical along the axis of said mass and whichexhibits vertical and transverse gradients whereby said mass is stronglyconstrained transversely but is free to move axially.
 4. A three-axisseismometer in accordance with claim 1 wherein said third mass is aferromagnetic body so oriented within a magnetic field as to besupported by a combination of magnetic attraction and magneticrepulsion.
 5. A three-axis seismometer in accordance with claim 1wherein said vertically oriented mass is mounted in a verticallypositioned rod member and diamagnetic bearings are associated with thetwo ends of said rod member thereby to maintain it in essentiallyfrictionless alignment.
 6. A three-axis seismometer in accordance withclaim 1 wherein said masses have light-obstructing means associated withone end thereof and each of said detecting means comprises a source ofradiation and a dual photocell positioned to respond to movement of saidlight-obstructing means.
 7. A three-axis seismometer in accordance withclaim 1 wherein said feedback means associated with said first andsecond masses comprise means to adjust the position of support platformmeans on which said first and second magnetic field generating means aremounted relative to said diamagnetic masses.
 8. A three-axis seismometerin accordance with claim 7 wherein said means to adjust the position ofsaid support platform means comprises piezoelectric crystals interposedbetween said platform means and base means in contact with the ground,the seismic motion of which is to be sensed.
 9. A three-axis seismometerin accordance with claim 1 wherein said feedback means associated withsaid masses comprises means to force said masses to return to apredetermined position relative to said magnetic field generating means.10. A three-axis seismometer in accordance with claim 1 wherein saidfeedback means associated with said masses comprises means to change theflux pattern in the magnetic field generating means whereby saidmagnetic masses are returned to a predetermined position relative tosaid magnetic field-generating means.
 11. A three-axis seismometer inaccordance with claim 1 including recording means adapted to record theseismic motion of each of said masses as a function of said amount ofconstraint required to contain said relative motion between each of saidmasses and its magnetic field generating means.
 12. A three-axisseismometer, comprising in combination a. first and second horizontallyoriented diamagnetic masses arranged to move in directions perpendicularto each other; b. first and second magnetic field-generating meansadapted to support said first and second diamagnetic masses foressentially only axial movement; c. a third vertically oriented mass; d.a third magnetic field-generating means adapted to support said thirdmass for essentially only axial movement; e. separate detecting meansassociated with each of said masses adapted to sense relative seismicmotion between said mass and said magnetic field generating means and toproduce a signal; f. separate feedback means associated with each ofsaid masses and its respective magnetic field-generating means andadapted to constrain said relative motion between said mass and itsassociated magnetic field-generating means; g. separatefeedback-actuating means adapted to receive said signal and to actuatesaid feedback means whereby said relative motion between said mass andits associated magnetic field-generating means is constrained; h. meansto record said signals, which are proportional to said seismic motions,as a function of time; i. an outer cylindrical housing; and j. first,second, and third inner cylindrical housings in axial alignment locatedin connected spaced relationship within said outer cylindrical housing,each of said inner housings containing its respective diamagnetic mass,magnetic field-generating Means, detecting means and feedback means. 13.A three-axis seismometer in accordance with claim 12 wherein saidfeedback means includes means to provide magnetic, electrostatic,inertial or gravitational force to effect the required constraint.
 14. Athree-axis seismometer in accordance with claim 12 including remotelyactivatable levelling means associated with said first and secondmagnetic field-generating means.
 15. A three-axis seismometer inaccordance with claim 12 wherein said feedback means comprise means toforce said masses to return to a predetermined position relative to saidmagnetic field-generating means.
 16. A three-axis seismometer inaccordance with claim 15 wherein said force is magnetic.
 17. Athree-axis seismometer in accordance with claim 12 includingperiod-adjustment coils associated with said third mass and means tosupply DC current to said coils.
 18. A three-axis seismometer inaccordance with claim 12 wherein said feedback means associated withsaid third mass comprise motor coils connected in series opposition. 19.A three-axis seismometer in accordance with claim 12 wherein said meansto record said signals include filter means adapted to filter outunwanted frequencies from said signal.